This invention relates generally to the treatment of radioactive waste solutions and, more particularly, to a process for immobilizing radioactive boric acid liquid wastes.
The safe and effective management of low-level radioactive waste is one of the most critical problems facing the nuclear industry today. Generally, low-level radioactive wastes are those wastes suitable for disposal in a near-surface facility and include routine wastes from reactor plants, radioactive wastes from hospitals and research institutions, and most industry-generated radioactive wastes. In order to meet present nuclear waste burial requirements under the Code of Federal Regulations as specified in 10 CFR 61, any radioactive waste solutions must be immobilized prior to such near-surface burial. The liquid wastes, or wastes continuing liquid, must be converted into a form that contains as little free standing and noncorrosive liquid as is reasonably achievable, but in no case shall the liquid exceed 1% of the volume of the waste.
One of the more common radioactive wastes that must be immobilized is spent boric acid aqueous solutions resulting from the boric acid utilized in the reactor control process of pressurized water reactor (PWR) facilities. A typical operating PWR can generate about 50,000 gallons of boric acid evaporator bottoms per year. A conventional method of treating such wastes, i.e., boric acid liquid wastes, is to neutralize them with sodium hydroxide and then concentrate the same in an evaporator to typically 10 to 12% solids. The resulting sodium borate slurries are then immobilized, usually in cement or urea-formaldehyde.
While these known processes have served their intended purposes, they have not been entirely satisfactory. For example, the immobilization of boric acid and borates in portland cement poses problems since borates retard setting of the cement and tend to destroy the hydraulic hardening characteristics of the cement. At present, there are limitations on the weight percent boric acid that can be solidified in portland cement and still meet transportation and waste burial requirements. Specifically, up to 16 weight percent boric acid is the maximum that can be solidified in portland cement under ordinary conditions without additives and the like. Also, this cement immobilization process generates as much as three times the original volume in solidified material, posing greater safety hazards and materially increasing haulage and burial costs.
The immobilization of boric acid and borates in urea-formaldehyde can cause container corrosion problems as a result of the acid catalyst employed therein. Moreover, a significant amount of "free water" can accumulate during setting and must be removed or solidified prior to burial. In light of the above, it can be appreciated that a need exists for converting boric acid liquid wastes into a more stable structural form possessing only negligible, if any, free standing liquid and for reducing the volume of such wastes while providing adequate radionuclide containment.
Accordingly, it is a primary object of the present invention to obviate the above noted shortcomings by providing a new and useful method of immobilizing radioactive boric acid liquid wastes in a manner achieving a substantial volume reduction of such wastes.
It is another object of this invention to convert the foregoing radioactive waste solutions into a stable, solid form providing improved radionuclide containment.
It is a further object of the present invention to convert boric acid radioactive waste solutions into an inert, insoluble glass product having optimum mechanical strength properties, and which is unaffected by moisture and other inadvertent intruders.
The foregoing and other objects, advantages, and characterizing features of the present invention will become clearly apparent from the ensuing detailed description of an illustrative embodiment thereof along with the accompanying drawing.